There you have it. Immediate dead ball. Runner is out for his act of interference. BR gets 1st.
I would only call a double play if I felt the interference was intentional or that the double play was obvious... and then it would be the runner closest to home (might be the BR if that was the only other runner.)
Double plays are rarely obvious. There are a lot of other elements that can influence the double play. Was there a runner coming into 2nd? Did the SS field the ball close to 2nd? Was there going to be a need for the SS to relay the ball to F4? In what part of the fielding action did the tangled feet occur? I think you are safest with only calling the interfering runner out.
